For our DAD/miniDAD project a few years ago, I came up with a solution of
how to configure Exim to use a smarthost if configured in LDAP. There
wasn't anything really revolutionary about that, but I found it quite nifty.
So much, that I expanded that a bit for my Exim and LDAP presentation
recently, and even showed how to create a specific schema for storing similar
values.

The true beauty of having an application (in this case the Exim MTA) pull
its configuration out of a directory shows itself when you then actually use
it.

Reconfiguring a dozen servers on the fly (ok, not a full dozen, but
close) was a cinch. One ldapmodify later, they all sent their mail somewhere
else. (In a controlled manner. :-) )